1. Field of the Invention
The present invention relates to an electrophotographic toner which is used in electrophotographic apparatuses that utilize an electrophotographic process, such as copying machines, printers, facsimiles, and the like, a method of manufacturing the same, an electrophotographic developer, and an image-forming method.
2. Description of the Related Art
As described in Japanese Patent Application Publication (JP-B) No. 42-23910 and the like, a number of methods are known as electrophotographic methods. In general, a latent image is formed electrically by one of various means on the surface of a photorecepter (latent image holding material) which utilizes a photoconductive substance. The formed latent image is developed using a toner, and thus a toner image is formed. Thereafter, the toner image on the surface of the photorecepter is transferred onto the surface of a transfer material such as paper or the like via or not via an intermediate transfer material. The transferred image is subjected to a plurality of fixing processes such as heating, pressurizing, heat-pressurizing, solvent vapor, and the like, such that a fixed image is formed. Toner which remains on the surface of the photorecepter is cleaned by various methods as necessary and is subjected to the above-described plurality of processes again.
As a fixing technique for fixing a transferred image which has been transferred onto the surface of a transfer material, a heat roll fixing method is generally known. In this method, a transfer material, onto which a toner image has been transferred, is inserted and fixed between a pair of rolls which is formed by a heat roll and a pressure roll. Further, as the same type of technique, a technique of replacing one or both of the rolls with a belt (belts) is also known. In these techniques, compared to other fixing methods, a fast fixed image is obtained quickly, energy efficiency is high, and harm to the environment due to volatilization of a solvent or the like is small.
On the other hand, in order to reduce the amount of energy used by a copying machine or printer, a technique for fixing a toner with less energy is desired. Accordingly, demand for an electrophotographic toner which can be fixed at a lower temperature is strong.
As a means of lowering the fixing temperature of a toner, a technique of lowering the glass transition temperature of a toner resin (binder resin) is generally effected.
However, if the glass transition temperature is too low, flocculation (xe2x80x9cblockingxe2x80x9d) of fine particles easily occurs and storability of toner as the fixed image is lost. As a result, the minimum glass transition temperature is 60xc2x0 C. in practice. The glass transition temperature is a design point of many toner resins which are available at present. There is a problem in that a toner which can be fixed at an even lower temperature cannot be obtained by simply using methods of lowering glass transition temperature. Moreover, the fixing temperature can also be lowered using a plasticizer. However, there is a drawback in that blocking occurs during storage of a toner or in the developing machine.
As a means of preventing blocking, having image storability up to 60xc2x0 C., and having low temperature fixability, a technique of using a crystalline resin as a binder resin for forming a toner has been considered and has been known (JP-B-56-13943 and the like). Further, a technique of using a crystalline resin for the purpose of preventing offset (JP-B-62-39428), of pressure fixing (JP-B-63-25335), and the like has been known.
The above disclosed techniques have problems. For example, in the technique disclosed in JP-B-56-13943, a polymer which has an alkyl group side chain having 14 or more carbon atoms is used in a toner. The melting point of the polymer is as low as 62 to 66xc2x0 C. Because of the excessively low temperature, there is a problem of reliability of fine particles and images. Moreover, in crystalline resins described in JP-B-62-39428 and JP-B-63-25335, there is a problem in that fixing performances thereof with paper are not sufficient.
A crystalline resin, with which an improvement of fixability with a paper is sought, includes a polyester resin. A technique of using crystalline polyester resin for a toner is described in JP-B-62-39428. In this technique, an amorphous polyester resin having a glass transition temperature of 40xc2x0 C. or more and a crystalline polyester resin having a melting point of from 130 to 200xc2x0 C. are mixed and used.
This technique provides excellent pulverizing ability and blocking resistance. However, since the melting point of the crystalline polyester resin is high, there is a drawback in that fixability at lower temperatures cannot be achieved.
In order to solve the above-described drawback, a technique of using a toner in which a crystalline resin having a melting point of 110xc2x0 C. or less is mixed with an amorphous resin (JP-B-4-30014) is proposed.
However, if the amorphous resin is mixed with the crystalline resin, there are practical problems such as lowering of the melting point of the toner, occurrence of toner blocking, deterioration of the storability of an image, and the like. Further, if an amount of amorphous resin component is large, characteristics of the amorphous resin component are greatly reflected. Accordingly, it is difficult to lower the fixing temperature of the toner more than tat of the conventional toner. As a result, the crystalline resin is used alone as a toner resin or, if the amorphous resin is mixed in, the amount of the amorphous resin has to be very small or there will be a practical problems.
As described above, it is desirable that a crystalline polyester resin is used for heat roll fixing as alone as possible. A technique of using crystalline polyester resin is described in Japanese Patent Application Laid-Open (JP-A) Nos. 4-120555, 4-239021, 5-165252, and the like. However, in these techniques, the crystalline polyester resin is a resin using alkylene glycol or alicyclic alcohol, which has few carbon atoms compared to the carboxylic acid component of terephthalic acid.
These polyester resins are described above as crystalline polyester resins. However, actually, these are partial crystalline polyester resins. As a result, a viscosity change of the toner (resin) with temperature is not sharp. Although there is no problem with blocking resistance and storability of an image, low temperature fixing cannot be achieved in heat roll fixing.
On the other hand, the present inventors have disclosed in the specification of JP-A-11-300158 that a toner which includes a crystalline polyester resin having a crosslinking structure as a main component has excellent blocking resistance, excellent storability of an image, and can realize low temperature fixing. However, in this toner as well, improvement of chargeability is further desired, especially in two-component charging with a carrier.
A subject of the present invention is to solve the above-described conventional problems and to achieve the following object. Namely, the object of the present invention is to provide an electrophotographic toner which has excellent blocking resistance, excellent storability of an image, excellent fixability at a low temperature and, further, excellent chargeability; a method of manufacturing the same; an electrophotographic developer; and an image-forming method.
The above-described first object is achieved in accordance with the following present invention. Namely, a first aspect of the present invention is an electrophotographic toner which comprises a binder resin and a colorant. The binder resin comprises a crystalline polyester resin as a main component, and an ester density M of the crystalline polyester resin, which is defined in Formula 1 as follows, is from 0.01 to 0.12:
M=K/Axe2x80x83xe2x80x83Formula 1 
wherein M denotes the ester density, K denotes a number of ester groups in a polymer, and A denotes a number of atoms constituting a high molecular chain of the polymer.
The second object is achieved in accordance with a following method for manufacturing an electrophotographic toner. The method comprises the steps of:
emulsifying a crystalline polyester resin; and
aggregating and coalescing the emulsified crystalline polyester resin,
adjusting the crystalline polyester resin to toner size, and
preparing an electrophotographic toner comprising a colorant and a binder resin, the binder resin including the crystalline polyester resin as a main component, and an ester density M of the crystalline polyester resin, which is defined in Formula 1 as follows, being from 0.01 to 0.12:
M=K/Axe2x80x83xe2x80x83Formula 1 
wherein M denotes the ester density, K denotes a number of ester groups in a polymer, and A denotes a number of atoms constituting a high molecular chain of the polymer.
Further, preferably, the crystalline polyester resin contains, as a copolycondensation component, a bivalent or more carboxylic acid having a sulfonic acid group.
The third object is achieved in accordance with a following method. The image forming method comprises the steps of:
forming an electrostatic latent image on a surface of a latent image holding material;
providing a developer comprising an electrophotographic toner including a colorant and a binder resin, and the binder resin comprising a crystalline polyester resin as a main component, and an ester density M of the crystalline polyester resin, which is defined in Formula 1 as follows, is from 0.01 to 0.12:
M=K/Axe2x80x83xe2x80x83Formula 1 
wherein M denotes the ester density, K denotes a number of ester groups in a polymer, and A denotes a number of atoms constituting a high molecular chain of the polymer;
forming a toner image from the electrostatic latent image formed on the surface of the latent image holding material by using the developer held on a developer holding member;
transferring the toner image formed on the surface of the latent image holding material onto a surface of a transfer material; and
heat-fixing the toner image transferred on the surface of the transfer material.
It is desirable that the above crystalline polyester resin is a straight chain aliphatic polyester resin. Further, it is desirable that a crystalline polyester-constituting component contains a dicarboxylic acid having at least one sulfonic acid group and/or a diol having at least one sulfonic acid group.